Reland of Add unit tests for RTCMTLVideoView. (patchset #1 id:1 of https://codereview.webrtc.org/2733953006/ )

Reason for revert:
Dependant CL has been relanded

Original issue's description:
> Revert of Add unit tests for RTCMTLVideoView. (patchset #6 id:100001 of https://codereview.webrtc.org/2723903003/ )
>
> Reason for revert:
> This CL depends on a reverted CL.
>
> Original issue's description:
> > Add unit tests for RTCMTLVideoView.
> >
> > To properly test the functionality,  following changes  were needed
> > - Make RTCMTLVideoView compiliable for all cpu architectures not just arm64.
> > This is needed so that the test can run on any device and on simulator as well.
> > - Refactor RTCMTLVideoView to have  mockable class methods.
> > The unittest class, RTCMTLVideoViewTests was designed to provide easy transition
> > to XCTest when the time comes for that.
> > To transition to XCTest it would suffice to inherit from XCTestCase and remove
> > the gtest methods.
> >
> > BUG=webrtc:7079
> >
> > Review-Url: https://codereview.webrtc.org/2723903003
> > Cr-Commit-Position: refs/heads/master@{#17014}
> > Committed: 0ebe0199ac
>
> TBR=magjed@webrtc.org,denicija@webrtc.org
> # Not skipping CQ checks because original CL landed more than 1 days ago.
> BUG=webrtc:7079
>
> Review-Url: https://codereview.webrtc.org/2733953006
> Cr-Commit-Position: refs/heads/master@{#17118}
> Committed: 96d91524fa

TBR=magjed@webrtc.org,denicija@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7079

Review-Url: https://codereview.webrtc.org/2743663002
Cr-Commit-Position: refs/heads/master@{#17140}

webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLNV12Renderer.h

@@ -24,12 +24,6 @@
  * @param frame The frame to be rendered.
  */
 - (void)drawFrame:(RTCVideoFrame *)frame;
-@end
-
-/**
- * Implementation of RTCMTLRenderer protocol for rendering native nv12 video frames.
- */
-@interface RTCMTLNV12Renderer : NSObject <RTCMTLRenderer>
 
 /**
  * Sets the provided view as rendering destination if possible.
@@ -37,5 +31,13 @@
  * If not possible method returns NO and callers of the method are responisble for performing
  * cleanups.
  */
-- (BOOL)addRenderingDestination:(__kindof MTKView *)view;
+- (BOOL)addRenderingDestination:(__kindof UIView *)view;
+
+@end
+
+/**
+ * Implementation of RTCMTLRenderer protocol for rendering native nv12 video frames.
+ */
+@interface RTCMTLNV12Renderer : NSObject <RTCMTLRenderer>
+
 @end

webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLVideoView.m

@@ -18,15 +18,18 @@
 
 #import "RTCMTLNV12Renderer.h"
 
+// To avoid unreconized symbol linker errors, we're taking advantage of the objc runtime.
+// Linking errors occur when compiling for architectures that don't support Metal.
+#define MTKViewClass NSClassFromString(@"MTKView")
+#define RTCMTLNV12RendererClass NSClassFromString(@"RTCMTLNV12Renderer")
+
 @interface RTCMTLVideoView () <MTKViewDelegate>
-@property(nonatomic, strong) id<RTCMTLRenderer> renderer;
+@property(nonatomic, strong) RTCMTLNV12Renderer *renderer;
 @property(nonatomic, strong) MTKView *metalView;
 @property(atomic, strong) RTCVideoFrame *videoFrame;
 @end
 
-@implementation RTCMTLVideoView {
-  id<RTCMTLRenderer> _renderer;
-}
+@implementation RTCMTLVideoView
 
 @synthesize renderer = _renderer;
 @synthesize metalView = _metalView;
@@ -58,11 +61,41 @@
 #endif
 }
 
++ (MTKView *)createMetalView:(CGRect)frame {
+  MTKView *view = [[MTKViewClass alloc] initWithFrame:frame];
+  return view;
+}
+
++ (RTCMTLNV12Renderer *)createMetalRenderer {
+  RTCMTLNV12Renderer *renderer = [[RTCMTLNV12RendererClass alloc] init];
+  return renderer;
+}
+
 - (void)configure {
-  if ([RTCMTLVideoView isMetalAvailable]) {
-    _metalView = [[MTKView alloc] initWithFrame:self.bounds];
-    [self addSubview:_metalView];
+  if (![RTCMTLVideoView isMetalAvailable]) {
+    RTCLog("Metal unavailable");
+    return;
+  }
+
+  _metalView = [RTCMTLVideoView createMetalView:self.bounds];
+  _renderer = [RTCMTLVideoView createMetalRenderer];
+
+  if ([self configureMetalRenderer]) {
+    [self configureMetalView];
+  } else {
+    _renderer = nil;
+    RTCLogError("Metal configuration falied.");
+  }
+}
+
+- (BOOL)configureMetalRenderer {
+  return [_renderer addRenderingDestination:_metalView];
+}
+
+- (void)configureMetalView {
+  if (_metalView) {
     _metalView.delegate = self;
+    [self addSubview:_metalView];
     _metalView.contentMode = UIViewContentModeScaleAspectFit;
     _metalView.translatesAutoresizingMaskIntoConstraints = NO;
     UILayoutGuide *margins = self.layoutMarginsGuide;
@@ -70,20 +103,14 @@
     [_metalView.bottomAnchor constraintEqualToAnchor:margins.bottomAnchor].active = YES;
     [_metalView.leftAnchor constraintEqualToAnchor:margins.leftAnchor].active = YES;
     [_metalView.rightAnchor constraintEqualToAnchor:margins.rightAnchor].active = YES;
-
-    _renderer = [[RTCMTLNV12Renderer alloc] init];
-    if (![(RTCMTLNV12Renderer *)_renderer addRenderingDestination:_metalView]) {
-      _renderer = nil;
-    };
-  } else {
-    RTCLogError("Metal configuration falied.");
   }
 }
+
 #pragma mark - MTKViewDelegate methods
 
 - (void)drawInMTKView:(nonnull MTKView *)view {
   NSAssert(view == self.metalView, @"Receiving draw callbacks from foreign instance.");
-  [_renderer drawFrame:self.videoFrame];
+  [self.renderer drawFrame:self.videoFrame];
 }
 
 - (void)mtkView:(MTKView *)view drawableSizeWillChange:(CGSize)size {
@@ -92,8 +119,8 @@
 #pragma mark - RTCVideoRenderer
 
 - (void)setSize:(CGSize)size {
-  _metalView.drawableSize = size;
-  [_metalView draw];
+  self.metalView.drawableSize = size;
+  [self.metalView draw];
 }
 
 - (void)renderFrame:(nullable RTCVideoFrame *)frame {